A navigation system performs travel guidance for enabling a user to easily and quickly reach the selected destination. A typical example is a vehicle navigation system where a vehicle is equipped with a navigation function to guide a driver to a destination through a calculated route. Such a navigation system detects the position of the user's vehicle, and reads out map data pertaining to an area at the current vehicle position from a data storage medium, for example, a DVD (digital versatile disc), a hard disc, etc.
Typically, the navigation system displays a map image on a monitor screen while superimposing thereon a mark representing the current location of the vehicle of the user. A road on which the vehicle is running will be highlighted which typically means a calculated route to the destination. Many recent navigation systems are designed to be able to display a three-dimensional map image on which roads, buildings, mountains, fields, and other objects on the surface of the ground are displayed in a three-dimensional manner.
Such a three-dimensional map image is useful for helping the user to navigate through geographic areas by providing views similar to the actual surrounding images. However, a three-dimensional map image on a navigation system screen involves drawbacks as well. While it offers views that look similar to what the driver actually sees though the windscreen, some elements, such as large buildings or natural objects can obstruct part of the three-dimensional view. For instance, a mountain or a large building can obstruct the user's view such that the user is unable to see what is ahead on the road.
FIGS. 1A-1C are diagrams showing display examples of three-dimensional map image which involve the situations where the problems described above have occurred. In the example of FIG. 1A, a view of the road 5 on which the user's vehicle is running is obstructed ahead by buildings 7 and 9. Because the three-dimensional map image typically provides a view that the user actually see through the wind shield of the vehicle, the road at the other side of the buildings are invisible.
In another example of FIG. 1B, a view of the road 15 is obstructed ahead by a mountain 17 and other surroundings since the road is curved and extended to the back side of the mountain 17. Thus, from the direction of the user, the road 15 becomes invisible because of the mountain 17. In a further example of FIG. 1C, a view of the road 25 is obstructed ahead by a cliff 27 and other surroundings. Again, because the road 15 is curved and extended to the back side of the cliff 27, it becomes invisible from the direction of the user's vehicle,
Here, a point where the road begins to lose its visibility due to the obstruction may also be referred to as a warning point or hidden point. In the example of FIG. 1C, a hidden point 31 is illustrated to indicated the point where the visibility is lost for the road 25. Because of such obstructions, the user may not be able to prepare for the hidden point, which may results in an accident. Therefore, there is a need of a new method and apparatus for a navigation system for finding a hidden point where visibility of a route will be lost on a three-dimensional map image, and producing a warning to notify the user the location of the hidden point, a shape of the route in the vicinity of the hidden point, etc.